30 ELEMENTS OF ELECTRICAL ENGINEERING. 



Substituting the value of z' 2 from (ii) and the value of z 3 from 

 (iii) in (i) we have : 



\ + 1.87 amperes 



whence from equations (ii) and (iii) we have : 

 i 2 = 0.86 amperes 

 z' 3 = -f 2.73 amperes 



21. Faraday's discovery. Induced electromotive force and in- 

 duced current. In 1831 Faraday discovered that a momentary 

 electric current is produced in a coil of wire when a magnet is 

 pushed into or withdrawn from the opening of the coil, or when 

 an iron rod around which the coil is wound is magnetized or 

 demagnetized. The motion of the magnet in the first case, or the 

 changing magnetism of the iron rod in the second case, produces 

 a momentary electromotive force in the coil, and this electromo- 

 tive force produces a momentary electric current in the coil if the 

 coil forms a portion of a closed circuit. Electromotive force and 

 electric current produced in this way are called induced electro- 

 motive force and induced current. 



22. Lenz's Law. An induced current always opposes the ac- 

 tion which produces it, and the work done in overcoming this 

 opposition is the work which is expended in producing the in- 

 duced current. 



Examples. (d) When a magnet is pushed into a coil the in- 

 duced current is in such a direction as to tend to push the mag- 

 net out of the coil ; more work must, therefore, be done in push- 

 ing the magnet into the coil than would be done if the circuit of 

 the coil were open so that no induced current could be produced ; 

 this extra work is the work which is expended in producing the 

 induced current. 



(ti) When an iron rod wound with wire is magnetized, the cur- 

 rent induced in the winding of wire opposes the magnetization, 

 and more work must be done to magnetize the rod than would 



